1. Field of the Invention
This invention relates to the sealing of alumina arc tubes for arc discharge lamps and in particular to high pressure sodium (HPS) vapor lamps.
2. Description of the Prior Art
Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps is of the order of about 50 to 100 Torr.
Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vaor pressure is in the order of a few microns. Low pressure sodium lamps have been in use for about 30 years but although efficient, they produce an unattractive monochromatic yellow light. The color of light from HPS lamps is considerably improved over that from low pressure sodium lamps.
HPS lamps generally comprise an alumina ceramic arc tube containing a fill including sodium, mercury and an inert gas. Disposed at the ends of the arc tubes are metal electrodes and the ends of the arc tube are sealed with ceramic to metal seals.
A common method of manufacture of the arc tube, shown in U.S. Pat. No. 3,608,437, involves the use of a cylindrical sealing element of densely sintered aluminum oxide which is sintered to the inner wall at the end of a polycrystalline alumina arc tube and which contains an aperture for a cylindrical current lead-in tube. However, such a cylindrical sealing element cannot be used with a single crystal alumina arc tube since it cannot be sintered thereto in a gastight manner.